This article presents the study of a pathology or deterioration that is somewhat peculiar and, therefore, little studied: the corrosion of the gilding of the plasterwork of the monumental complex of the Alhambra. This deterioration manifests itself through a purple formation in the gilding. The researchers present a successful multi-analytical methodology to determine both the nature and the distribution of the corrosion products present in the gilding. In this case, the tin layer applied before gilding assisted the subsequent formation of gold nanospheres deposited on the surface from the original gold layer. The purple colour observed would be associated with what is technically known as surface plasmon resonance, which, through the presence of metallic nanoparticles (gold in this case), causes a tonal change in the surface where they are deposited. In this case, the addition of gold nanoparticles has formed spontaneously, but many researchers are currently taking advantage of this phenomenon to, for example, tune or modify the colour of materials to taste, detect nanoplastics in the environment by means of a colour change due to their interaction with metal nanoparticles, etc. 

This work demonstrates that the use of microscopic techniques and analysis at low lateral resolution is crucial for detecting and analysing particles at the micro and nanometric scale. It also highlights the coupling of different analytical techniques in order to determine the elemental and molecular nature. This coupling of instruments is crucial if one wants to determine the nature of randomly distributed phases or decay products at the microscopic scale. Therefore, this article shows that, although it is a good proposal for research in the conservation of cultural heritage, it is necessary to have state-of-the-art instrumentation in order to be able to respond to the problems that arise in this field.  

In addition, this work shows that, on many occasions, it is crucial to be able to have a sample available for study in the laboratory. Taking into account the historical-artistic value of heritage assets, it can sometimes happen that sampling is restricted or even unauthorised. Nowadays, there are numerous portable alternatives based on non-invasive analysis techniques, which allow direct analysis of surfaces or materials. In some cases, these techniques are sufficient to obtain conclusive results. However, when the material or object to be analysed is structured in different layers on a microscopic scale (as is the case here) and the aim is to study what happens in each of them and how they affect the rest, it is essential to take a sample. Without a stratigraphic study, the authors of this work would never have been able to propose the chemical deterioration route described to explain the pathology studied. 

This study also shows that, on many occasions, it is not necessarily anthropic factors or sources caused by humans that promote the deterioration of cultural heritage. In this case, as the authors point out, humidity itself and the diffuse impact of marine aerosol may be sufficient to assist the deterioration process described. 

For all these reasons, this article presents a clear example of how science applied to cultural heritage is crucial to understand the mechanisms of deterioration and the causes that give rise to them. This information is extremely valuable in order to subsequently develop appropriate conservation strategies to conserve the jewels of our heritage such as, in this case, the Alhambra. 

In 2011, the European Commission proposed a definition of "nanomaterial" as a material containing particles with one or more external dimensions between 1 and 100 nanometres. A nanometre is one million times smaller than a millimetre. 

Although talk of nanomaterials research has become more widespread in recent years, especially because of the interest in many of their applications, mankind has been using nanomaterials since ancient times, whether naturally or artificially produced. Scientific characterisation techniques, which have been refined and have gained in resolution in recent decades, have allowed us to gain in-depth knowledge of their composition and structure and thus understand the formation mechanisms of these historical nanomaterials, surprising us with the technology that was used centuries ago. For example, some research dates the first production of ruby glass, due to the presence of gold nanoparticles inside it, to the 7th century BCE, based on Assyrian tablets that speak of the production of "an artificial coral". 

The contribution made by this research on the alteration of the gold decorations of the Alhambra shows us, on the one hand, the complexity of the work carried out to create these extraordinary works of art and, on the other, provides arguments to explain how they degrade under certain conditions to produce gold nanoparticles. The study has been able to combine the use of several high-resolution analytical microscopy techniques, complementing each other, to gain an in-depth understanding of the result of the corrosion of the decorations and to propose possible formation mechanisms that explain the appearance of these "strange" purple colourations. Despite the "nobility" of gold, certain situations such as, in this case, humidity, the presence of a high chloride content and dirt lead to its slow decomposition, with oxidation and subsequent reduction phenomena that have seriously affected the work of art. The case of the colour we see in gold, like that of other metals, is very curious because it depends on the size of its particles, which can be red or purple if they are nanometres in size, or golden when they form sheets. 

The results of this research highlight the importance of proper conservation of our heritage, avoiding conditions that could alter it, especially at times like the present, when extreme temperatures or high levels of pollution can be common. They also provide, for the first time, the necessary keys to understand the appearance of these alterations and, therefore, to deal with possible restorations and proper conservation of this and other works of art with gold leaf decorations.